so you can't really fire someone for a sick day, but the reality is that most people in such jobs are either on zero hours or agency contracts, so it's not being sacked, it's just not being given hours. And a tribunal isn't going to even look at your case as a short-timer, unless actual discrimination is involved.

in the first two years you can just be handed your notice without reason, no disciplinary required. Which is why employees of less than two years don't get statutory redundancy in the case of job losses.

However if you have evidence that the underlying reason is discrimination you can take them to tribunal.